The electromagnetic components of rotating electrical machinery generate heat which must be removed for normal operation. Typically, larger electrical machinery, such as electrical service generators or drive motors, are cooled by introducing a coolant fluid internally to the stator of the machine. For example, coolant fluid has been introduced internally into the stator of larger electrical machinery either by incorporating fluid ducts physically within the electromagnetic stator core, and circulating a coolant fluid therethrough, or by introducing a coolant fluid directly into hollow electrical conductors.
Smaller sized motors, such as integral or fractional horsepower motors, or larger sized motors that can be completely enclosed and thereby isolated from their environment of use, have sometimes been cooled externally by allowing air as a coolant to flow over the outer machinery frame. The forced flow of air over the external frame of the machinery necessitates that relatively large sized air passages are needed thereby mitigating against miniaturization.
Some commercially available drive motors, such as the totally enclosed fan cooled induction motors that have been sold by General Electric Company for more than one year prior to the filing of the present patent application, have been cooled by means of forced air flowing over the outer, and relatively low thermally conductive, frame of the motor to remove internally generated heat. Some of these previously sold motors have also included copper laminations within the stator core for purposes of providing a path of heat conductance to an outer region of the motor. As noted briefly above, however, these machines were provided with only relatively low thermal conductivity air-cooled frames, and hence did not include liquid-cooled high conductivity thermal collectors in thermal communication with the copper laminations.
The design of an adequate cooling system for high power density electrical machinery presents its own significant problems. In this regard, it is usually desired to have a high power density electrical machine that is both physically compact, yet has maximum power output. Various techniques have been employed in the past to maximize power output, such as by increasing the magnetic flux density, increasing the rate of cutting magnetic flux with conductors (high frequency), and increasing the rate of current flow in the conductors. As can be appreciated, all of these techniques contribute to the generation of significant internal heat within the machine that must be removed for normal operation.
In those instances where the size and/or weight of the electrical machine is not critical, a significant amount of the overall machine structure can be dedicated to the cooling system (e.g., the cooling ducts and passageways, peripheral support equipment, and the like) without detrimentally affecting the machine's performance characteristics. However, in high power density electrical machines, the specific performance ratings of the internal components must be maximized, while minimizing the available space for heat removal. Thus, cooling systems having significant spatial requirements cannot usually be tolerated for high power density electrical machinery. These competing design criteria--i.e., increased power density/decreased spatial requirements--must also be considered in the context of maintaining acceptable temperatures for reasonable component life and thermally induced stresses.
Previous high power density machinery represented by the High Speed/High Frequency Generators and the F-18 Generators sold by General Electric Company more than one year prior to the filing date of this application, have used liquid-cooled outer frames as replacements for coolant fluid passageways internally of the stator. However, the poor thermal conductance through both the armature magnetic core and the structural frame of these prior art generators create a significant temperature differential between the armature conductor and the cooling fluid. The conduction path thus limits the amount of heat that can be removed for a given maximum allowable internal temperature. As a result, the inefficiencies of the cooling system limit the power densities that may be achieved.
What has been needed, therefore, is a cooling system that allows the power output of electrical machines to be maximized, yet is conducive to machine miniaturization. It is towards attaining that need that the present invention is directed.